docs: def with_relation(self, relation_name=None):
          """
          if relation is not None, when fetch manytomany result, also
          fetch relation record and saved them to manytomany object,
          and named them as relation.
          
          If relation_name is not given, then default value is 'relation'
          """
          if not relation_name:
              relation_name = 'relation'
          if hasattr(self.modelb, relation_name):
              raise Error("The attribute name %s has already existed in Model %s!" % (relation_name, self.modelb.__name__))
          if not self.through_model:
              raise Error("Only with through style in ManyToMany supports with_relation function of Model %s!" % self.modelb.__name__)
          self.with_relation_name = relation_name
          return self